Following in Their Footprints: Cuticular Hydrocarbons as Overwintering Aggregation Site Markers in Hippodamia convergens

Abstract

The convergent lady beetle (Hippodamia convergens) forms large overwintering aggregations at revisited montane microsites far removed from their summer foraging grounds. Although orientation responses to visual and altitudinal features of habitat can explain the arrival of migrants at the general overwintering macrosite, the role that pheromones play in the accumulation of individuals in inconspicuous hibernacula microsites is not fully understood. Through two-choice bioassays and gas chromatography and mass spectrometry, we found that H. convergens orient towards hydrocarbons previously deposited on their walking surfaces by conspecifics. n-Tricosane (C23) is primarily responsible for this chemically-mediated orientation. Footprint extracts, as well as C23 alone, induce the eventual accumulation in the field of migrant H. convergens at artificial hibernacula, confirming their probable role as aggregation signals. Aggregations persisted over many days when footprint extracts were applied in conjunction with the previously identified 2-isobutyl-3-methoxypyrazine (IBMP) aggregation pheromone. The C23 hydrocarbon functions as a pheromone that interacts with responses to methoxypyrazines to effectively mediate formation of persistent aggregations of diapausing conspecifics at specific microsites. Also discussed is the potential effect that C23 has as a persistent scent marker in establishing the traditional use of hibernacula.

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Acknowledgments

We are grateful to Jocelyn Millar for access to chemical standards and GC/MS analyses. We also thank the James San Jacinto Mountain Reserve directors, Becca Fenwick and Jennifer Gee, for facilitating collection of beetles and cooperation in securing the test plots necessary for field bioassays. The research was supported in part by the Robert and Peggy van den Bosch Scholarship, and the Temecula Valley Wine Society.